In order to minimize the risk of contamination from a used syringe, it is common practice for used syringes to be deposited into heavy duty plastic bins which cannot be pierced by the needles. The bin and contents are then moved to a disposal facility at which they are incinerated. This means of disposal is of limited effect as it does not eliminate "needle stick" risk between needle use and disposal. Current disposal methods are also relatively expensive.
Numerous attempts have been made to design an acceptable syringe in which, after use, the needle is withdrawn into the body of the syringe and retained there in some manner. These designs are all directed to the same end of covering the needle after use to prevent inadvertent "needle stick" injuries with their attendant risk of cross-infection and to prevent reuse of the syringe. In many of these prior art arrangements the withdrawal of the needle into the body is entirely manual and requires the syringe user to remember to make some deliberate relative movement, normally between the plunger and the body, to effect withdrawal of the needle into the body of the syringe. Proposals have been made, as in Australian Patent Specifications 593,513, 594,634 and 35,676/89, to induce automatic withdrawal of the plunger into the body by the use of a helically coiled spring. This necessitates the use of additional and costly parts in the syringe and complicates its assembly.
In its first aspect the present invention is directed to an alternative arrangement for the automatic withdrawal of a syringe needle into the body of the syringe.
The above Prior art proposals for automatic withdrawal of the plunger have the disadvantage that immediately upon removal of the positive manual pressure holding the plunger in a depressed condition the spring will immediately stop movement of the plunger to its extended condition and simultaneously commence withdrawal of the needle into the body of the syringe. This could cause tissue tear and inadvertent and unwanted suction of blood into the syringe unless the operator consciously keeps the plunger depressed until the syringe is fully withdrawn from the patient's body. In a second aspect the present invention provides braking means to slow at least the initial rate of withdrawal of the needle into the body of the syringe.